This invention relates to a thermoplastic release coating or tapes and films using this coating.
There are widely varying chemistries and techniques used to provide for release properties from a film or other web backing used in conjunction with a pressure-sensitive adhesive. Premium release materials have low release values with respect to conventional pressure-sensitive adhesives generally less than 25 to 50 grams/25 mm. These types of release values can be obtained using catalyst cured release systems based on polydialkylsiloxane. Modified release is often desired, for certain applications, which typically requires release values ranging from 100 to 500 grams/25 mm, and occasionally even higher. Slightly modified release values have been obtained with these cured systems by cross-linking silicone with non-silicone or modifiers and/or use of mixed silicones or the addition of MQ silicate resin as a release modifier (generally at levels of greater than 20 percent). For example, U.S. Pat. No. 5,292,787 discloses a thermally curable polyorganosiloxane using a controlled release additive of an epoxy-functional siloxane used in conjunction with a vinyl-functional siloxane using suitable cross-linkers and hydroxylation catalysts. In the examples, the addition of about 30 percent of the epoxy functional siloxane approximately doubled the release values (Examples 3 and 6) depending on the degree of branching of the epoxy functional siloxane.
However, there is a often a desire to avoid these curable systems, particularly where manufacturing flexibility is desired. Systems that require curing often present many difficult problems, for example the components are often hard to maintain in a stable form, the release coatings and components are often subject to wide manufacturing and performance variability, moisture or gel contamination and other problems. A proposed alternative to curable systems is the use of copolymers having polyalkylsiloxane segments or blocks and so called hard segments or blocks. In these systems, physical crosslinking occurs by self-association of the higher Tg hard segments or blocks. Segmented or block copolymers of this type are described, for example, in European Patent Application No. 250248, which proposes a release coating comprising a copolymer with at least polyorganosiloxane segment(s) and polyurea and/or polyurethane segment(s). These segmented copolymers are obtained by a condensation reaction of amine terminated difunctional polyorganosiloxane and diisocyanate, with optional chain extenders such as diamines and/or dialcohols, which can be soft segment materials. Using this type of segmented copolymer, release values for the release coating can be changed by varying the relative ratio and size of the hard and/or soft segments (e.g., for the polyorganosiloxane soft segments and any soft segment chain extenders). For example, this European patent indicates that the relative amount of hard segments in the copolymer can range from about 15 percent to about 70 percent to provide various levels of release performance. These segmented copolymer compositions can be solvent or hot melt coated. It is also disclosed that when blended with an MQ resin (at 50:50) certain of the copolymers (e.g. those having elastomeric properties) can form pressure-sensitive adhesives. Although these copolymers are desirable in many respects for use as release coatings, the modification of the release properties of these segmented copolymers is difficult, without changing the constituent monomers, oligomers or the like. Particularly in a manufacturing setting where there is a need for the ability to have a simple and easily adjustable method for modifying the release characteristics of a given release coating on line without changing the nature of the components used (e.g. with a segmented copolymer changing its constituent segments, monomers etc.).
U.S. Pat. No. 3,957,724 describes a release material formed from a polyorganosiloxane containing active hydrogens which are reacted with polyisocyanates prepolymerized with polyols or polyamines to form a chain extended prepolymer. The formed release material is not thermoplastic due to extensive crosslinking which system shares the disadvantages of the cured systems discussed above.
U.S. Pat. No. 4,002,794 describes an organopolysiloxane material having mercapto-terminal groups which are reacted with diisocyanates to form release materials. The material obtained is described as coatable from a solvent and does not require curing in order to form a solid release coating. Also, the siloxane release materials are described as being capable of being intermixed with other film forming materials including epoxy resins, polyesters, polyamides, polyacrylates, polyurethanes, polyvinylchloride and like film forming resins.
Elastomeric polyorganosiloxane/polyurethanes copolymers are disclosed in U.S. Pat. No. 4,518,758, which patent indicates that poor elastomeric properties were obtained in prior described methods for forming these types of block copolymers due to formation of heterogeneity in the medium when reacting a macrodiol containing a long polysiloxane chain and a short polyisocyanate or a short polyol.
Other patents dealing with polyorganosiloxane block or segmented copolymers used as a film or other uses include the following.
U.S. Pat. No. 5,045,391 describes the use of silicone-polyurea elastomers on an image release sheet used in electrographic processes.
U.S. Pat. No. 4,758,601 describes forming a cellular polyurethane polyorganosiloxane elastomer using an organic polyisocyanate, polyhydroxyl, water and a chain extending agent having two or more sites with active hydrogen capable of bonding with an isocyanate.
Japanese Patent Application No. 1-126,389 describes a release agent formed using an organopolysiloxane reactive with hard segment comprising polyurethane, polyurea, polyamide, or polyester material. This material is similar to that described in U.S. Pat. No. 3,957,724 mentioned above in that it requires on line curing or crosslinking due to the multi-functionality of the organopolysiloxane and the isocyanate, optionally with multifunctional chain extending agents.
Japanese Patent Application No. 89-183987 (abstract) discloses a polyorganosiloxane formed into a block copolymer with an isocyanate terminated polyurea, the material is described as a film former which is useful for "selective gas permeation membranes".
U.K. Patent No. 214044 B describes a block copolymer of a polyorganosiloxane intermixed with a generic hard segment material having a Tg of greater than 37.degree. and a further soft segment of a polyethyleneoxide. The elastomeric material of this generic description allegedly has good biocompatibility and can be formed into films or other self-supporting materials for use in a biological setting.
U.S. Pat. No. 3,562,352 describes a polyorganosiloxane-polyurethane block copolymer formed by a condensation reaction between a vinylacetate group containing the polyorganosiloxane and a polyether or polyester urethane prepolymer.
U.S. Pat. No. 4,528,343 describes a copolymer of an organic polysiloxane with other non-silicone soft segments and polyurethane or polyurea hard segments to form an elastomeric material useful in conjunction with applications involving contact with blood. Similar biocompatible elastomers are described in U.S. Pat. No. 5,221,724 which reacts an aliphatic diisocyanate and an organic diisocyanate with an organopolysiloxane soft segment and a diol or diamine chain extender.